1. Field of the Invention
This invention relates to optical films, an more particularly to a method of depositing optical quality silica films, for example, in the manufacture of waveguides for optical Mux/Demux devices.
2. Description of Related Art
Optical devices such as optical Multiplexers (Mux) and Demultiplexers (Dmux) require extremely transparent optical quality silica waveguides in the 1.30 bi-directional narrow optical band and/or in the 1.55 μm video signal optical band which the International Telecommunications Union (ITU) recommends for Wavelength Division Multiplexing (WDM) transport network and for optical access networks using Fiber-To-The-Home (FTTH) technology.
Such waveguides are discussed in, for example, Uchida N., Passively aligned hibrid WDM module integrated with spot-size converter integrated laser diode for fibre-to-the-home, Electronic Letters, 32 (18), 1996; Inoue Y., Silica-based planar lightwave circuits for WDM systems, IEICE Trans. Electron., E80C (5), 1997; Inoue Y., PLC hybrid integrated WDM transceiver module for access networks, NTT Review, 9 (6), 1997; and Takahashi H., Arrayed-waveguide grating wavelength multiplexers for WDM systems, NTT Review, 10 (1), 1998;
These silica waveguides are basically composed of three films: a buffer, a core and a cladding. For reasons of simplicity, the buffer and the cladding are typically of same composition and typically have the same characteristics, i.e., the same refractive index at 1.55 wavelength (or 1.30 μm wavelength). In order to confine the 1.55 μm wavelength (and/or 1.30 μm wavelength) laser beam, the core must have a higher refractive index than the buffer (cladding) at 1.55 wavelength (and/or 1.30 μm wavelength). The required difference of refractive index between the core and the buffer (cladding) at 1.55 wavelength (and/or 1.30 μm wavelength) is called the ‘delta-n’. This ‘delta-n’ is one of the most important characteristics of silica waveguides. It is very difficult to fabricate optically transparent buffer (cladding) and core in the 1.55 μm wavelength (and/or 1.30 wavelength) optical region with a suitable ‘delta-n’.
Various technical approaches to obtain these high performance optically transparent silica waveguides have been tried in the prior art. A first approach is to use Flame Hydrolysis Deposition (FHD). This technique involves the fusion in hydrogen, oxygen and other gases of fine glass particles followed by some post-deposition anneals to 1200-1350° C. A second approach is the High Pressure Steam (HPS) technique. This technique involves the direct growth of silica films from silicon under an oxygen containing ambient at very high temperature followed by a very high temperature anneal at about 1000° C. A third approach is the Electron-Beam Vapor Deposition (EBVD) technique of quartz or silica at about 350° C. followed by very high temperature anneals at 1200° C.
Another is approach is to use the Plasma Enhanced Chemical Vapour Deposition (PECVD) technique. Such a technique is described in the following documents: Low temperature plasma chemical vapor deposition of silicon oxynitride thin-film waveguides, Applied Optics, 23 (16), 2744, 1984; Valette S., New integrated optical multiplexer-demultiplexer realized on silicon substrate, ECIO '87, 145, 1987; Grand G., Low-loss PECVD silica channel waveguides for optical communications, Electron. Lett., 26 (25), 2135, 1990; Bruno F., Plasma-enhanced chemical vapor deposition of low-loss SiON optical waveguides at 1.5-μm wavelength, Applied Optics, 30 (31), 4560, 1991; Kasper K., Rapid deposition of high-quality silicon-oxynitride waveguides, IEEE Trans. Photonics Tech. Lett., 3 (12), 1096, 1991; Lai Q., Simple technologies for fabrication of low-loss silica waveguides, Elec. Lett., 28 (11), 1000, 1992; Bulat E. S., Fabrication of waveguides using low-temperature plasma processing techniques, J. Vac. Sci. Technol. A 11 (4), 1268, 1993; Imoto K., High refractive index difference and low loss optical waveguide fabricated by low temperature processes, Electron. Lett., 29 (12), 1123, 1993; Bazylenko M. V., Fabrication of low-temperature PECVD channel waveguides with significantly improved loss in the 1.50-1.55 μm wavelength range, IEEE Ptotonics Tech. Lett., 7 (7), 774, 1995; Liu K., Hybrid optoelectronic digitally tunable receiver, SPIE, Vol 2402, 104, 1995; Yokohama S., Optical waveguide on silicon chips, J. Vac. Sci. Technol. A, 13 (3), 629, 1995; Hoffmann M., Low temperature, nitrogen doped waveguides on silicon with small core dimensions fabricated by PECVD/RIE, ECIO'95, 299, 1995; Bazylenko M. V., Pure and fluorine-doped silica films deposited in a hollow cathode reactor for integrated optic applications, J. Vac. Sci. Technol. A 14 (2), 336, 1996; Durandet A., Silica burried channel waveguides fabricated at low temperature using PECVD, Electronics Letters, 32 (4), 326, 1996; Poenar D., Optical properties of thin film silicon-compatible materials, Appl. Opt. 36 (21), 5122, 1997; Agnihotri O. P., Silicon oxynitride waveguides for optoelectronic integrated circuits, Jpn. J. Appl. Phys., 36, 6711, 1997; Boswell R. W., Deposition of silicon dioxide films using the helicon diffusion reactor for integrated optics applications, Plasma processing of semiconductors, Klumer Academic Publishers, 433, 1997; Hoffmann M., Low-loss fiber-matched low-temperature PECVD waveguides with small-core dimensions for optical communication systems, IEEE Photonics Tech. Lett., 9 (9), 1238, 1997; Pereyra I., High quality low temperature DPECVD silicon dioxide, J. Non-Crystalline Solids, 212, 225, 1997; Kenyon T., A luminescence study of silicon-rich silica and rare-earth doped silicon-rich silica, Fourth Int. Symp. Quantum Confinement Electrochemical Society, 97-11, 304, 1997; Alayo M., Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures, Thin Solid Films, 332,40, 1998; Bulla D., Deposition of thick TEOS PECVD silicon oxide films for integrated optical waveguide applications, Thin Solid Films, 334, 60, 1998; Canning J., Negative index gratings in germanosilica planar waveguides, Electron. Lett., 34 (4), 366, 1988; Valette S., State of the art of integrated optics technology at LETI for achieving passive optical components, J. of Modern Optics, 35 (6), 993, 1988; Ojha S., Simple method of fabricating polarization-insensitive and very low crosstalk AWG grating devices, Electron. Lett., 34 (1), 78, 1998; Johnson C., Thermal annealing of waveguides formed by ion implantation of silica-on-Si, Nuclear Instruments and Methods in Physics Research, B141, 670, 1998; Ridder R., Silicon oxynitride planar waveguiding structures for application in optical communication, IEEE J. of Sel. Top. In Quantum Electron., 4 (6), 930, 1998; Germann R., Silicon-oxynitride films for optical waveguide applications, 195th meeting of the Electrochemical Society, 99-1, May 1999, Abstract 137, 1999; Worhoff K., Plasma enhanced cyhemical vapor deposition silicon oxynitride optimized for application in integrated optics, Sensors and Actuators, 74, 9, 1999; and Offrein B., Wavelength tunable optical add-after-drop filter with flat passband for WDM networks, IEEE Photonics Tech. Lett., 11 (2), 239, 1999.
A comparison of these various PECVD techniques is summarised in the following table.
Refractive IndexPublicationPECVD ReactionControl MethodValette S., 1987UnknownP dopingValette S., 1988UnknownP dopingGrand G., 1990UnknownP dopingLiu K., 1995UnknownContent in Si, POjha S., 1998UnknownGe, B, or P dopingCanning J., 1998UnknownGe dopingBulla D., 1998TEOSTEOSJohnson C., 1998SiH4 + O2Si ion ImplantationBazylenko M. V., 1995SiH4 + O2 + CF4SiH4/O2/CF4 flow ratioBazylenko M. V., 1995SiH4 + O2 + CF4SiH4/O2/CF4 flow ratioBazylenko M. V., 1996SiH4 + O2 + CF4SiH4/O2/CF4 flow ratioDurandet A., 1996SiH4 + O2 + CF4SiH4/O2/CF4 flow ratioBoswell R. W., 1997SiH4 + O2 + CF4SiH4/O2/CF4 flow ratioKapser K., 1991SiH4 + N2OSiH4/N2O flow ratioLai Q., 1992SiH4 + N2OSiH4/N2O flow ratioPereyra I., 1997SiH4 + N2OSiH4/N2O flow ratioAlayo M., 1998SiH4 + N2OSiH4/N2O flow ratioImoto K., 1993SiH4 + N2O + N2SiH4/N2O/N2 flow ratioKenyon T., 1997SiH4 + N2O + ArSiH4/N2O/Ar flow ratioLam D. K. W., 1984SiH4 + N2O + NH3SiH4/N2O/NH3 flow ratioBruno F., 1991SiH4 + N2O + NH3SiH4/N2O/NH3 flow ratioYokohama S., 1995SiH4 + N2O + NH3SiH4/N2O/NH3 flow ratioAgnihotri O. P., 1997SiH4 + N2O + NH3SiH4/N2O/NH3 flow ratioGermann R., 1999SiH4 + N2O + NH3UnknownOffrein B., 1999SiH4 + N2O + NH3UnknownHoffmann M., 1995SiH4 + N2O + NH3 + ArSiH4/N2O/NH3/Ar flow ratioHoffmann M., 1997SiH4 + N2O + NH3 + ArSiH4/N2O/NH3/Ar flow ratioPoenar D., 1997SiH4 + N2O + NH3 + N2SiH4/N2O/NH3/N2 flow ratioRidder R., 1998SiH4 + N2O + NH3 + N2SiH4/N2O/NH3/Ar flow ratioWorhoff K., 1999SiH4 + N2O + NH3 + N2SiH4/N2O/NH3/N2 flow ratioBulat E.S., 1993SiH4 + N2O + N2 + O4 + CF4SiH4/N2O/N2/O2/CF4 flow ratioThis Patent ApplicationSiH4 + N2O + PH3 + N2Patented Method
This table compares methods that have been tried to modify the refractive index of the buffer (cladding) and core while trying to reduce their optical absorption. The various techniques can be grouped into the following main categories: PECVD using unknown chemicals coupled with unknown boron (B) and/or phosphorus (P) chemicals to adjust the refractive index of the silica films; PECVD using TEOS coupled with unknown means of adjusting the refractive index of the silica films; PECVD using oxidation of SiH4 with O2 coupled with unknown means of adjusting the refractive index of the silica films; PECVD using oxidation of SiH4 with O2 coupled with CF4 (SiH4/O2/CF4 flow ratio) to adjust the refractive index of the silica films; PECVD using oxidation of SiH4 with N2O coupled with N2O (SiH4/N2O flow ratio) to adjust the refractive index of the silica films; PECVD using oxidation of SiH4 with N2O coupled with N2O and N2 (SiH4/N2O/N2 flow ratio) to adjust the refractive index of the silica films; PECVD using oxidation of SiH4 with N2O coupled with N2O and Ar (SiH4/N2O/Ar flow ratio) to adjust the refractive index of the silica films; and PECVD using oxidation of SiH4 with N2O coupled with N2O and NH3 (SiH4/N2O/NH3 flow ratio) to adjust the refractive index of the silica films.
None of these prior art techniques satisfactorily addresses the problem of achieving high quality films with the ability to create the desired difference in refractive index between adjacent films, for example, forming the core and cladding layers of an optical waveguide.